In the oilfield service industry and specifically in the division known as the wellhead isolation tool service industry, several of the wellhead isolation tools in use are described in: Bullen, A Well Tree Saver, Canadian Patent No. 1,094,945, U.S. Pat. No. 4,241,786, McLeod, an Insertion Drive System for Tree Savers, Canadian Pat. No. 1,222,204, U.S. Pat. No. 4,632,183, Dallas-Garner, Wellhead Isolation Tool and Setting Device and Method of Using Same, Canadian Pat. No. 1,267,078, U.S. Pat. No. 4,867,243, Cummins, (Assigned to Halliburton Co.), a Wellhead Isolation Tool and Method of Use Thereof, U.S. Pat. No. 3,830,304. The purpose of these tools is to insert a mandrel through a wellhead and into the tubing or casing therein to allow the high pressure fluid to be injected and bypass the wellhead configuration. (For this discussion, we will refer only to the tubing although the same sealing systems which will be described have been used to seal in the well casing.) The difference seen in the isolation tools is mainly in the method used to push the mandrel into the wellhead. The end of the mandrel which enters the tubing has a sealing nipple on it that is generally referred to as a nipple, sealing means, packing or elastomer. Such sealing means is mentioned in Bullen, and five such nipples or sealing means which are in common use today in the isolation tool industry are shown in cross section as existing practice in FIGS. 1A to 7A. The various sealing shapes of the elastomers under pressure are shown in FIGS. 1B to 7B.
FIG. 1A shows Pitts, a Tree Saver Packer Cup, U.S. Pat. No. 4,023,814 (Assigned to Dow Chemical Co.). This packer cup has the elastomer 101 bonded at 102 to the nipple body 103. In FIG. 1B the elastomer is shown as a friction fit at 104 in the tubing 105 and is under pressure 106.
FIG. 2A, from Oliver, a Wellhead Isolation Tool, U.S. Pat. No. 4,111,261 (assigned to Halliburton) shows the elastomer 201 bonded at 202 to the nipple body 203. FIG. 2B shows the elastomer expanded by pressure 204 against the wall 205 of the tubing 206. FIG. 3A shows a sealing nipple from the same patent with a moveable primary seal element 301 including an O-ring seal and carrier 302. The primary seal element will move against and expand a packer ring 303. FIG. 3B shows this movement and expansion at 305 and 306 in the tubing 304 due to the pressure 307. The packer ring will collapse to its original shape when pressure is removed.
FIG. 4A shows McLeod, a Nipple Insert, Canada Pat. No. 1,169,766, U.S. Pat. No. 4,601,494, and has the elastomer seal 401 bonded at 402 to the nipple body 403 and the bond protected from the treating pressures by a tapered steel conical insert 404. In FIG. 4B it is shown as a friction fit at 405 in the tubing 406 under pressure 407.
FIG. 5A shows a later type of sealing nipple, from Sutherland-Wenger, a Wellhead Isolation Tool Nipple, Canadian Pat. No. 1,272,684. This sealing nipple is characterized by a protected moveable sealing element 501 which is moved out of the sealing cup 510 by pressure from the well through ports 502. An improvement by the inventors Sutherland-Wenger not described in the existing patent is presented and has the moveable sealing element backed by a split steel ring 503 on a conical seat 504 of the nipple body 505. As shown in FIG. 5B, this split steel ring expands due to the pressure 506 on the moveable sealing element to contact at 507 the inside of the tubing 508 and prevents the sealing element from being extruded up the annulus 509 formed by the nipple body and tubing. The moveable seal element will collapse to the original shape when pressure is removed.
FIG. 6A shows a version of a sealing nipple used by Arrowhead and FIG. 7A a version used by Dow Schlumberger which are somewhat like that described in Oliver (Halliburton), FIG. 3A. They feature two sealing elements noted as the primary sealing element 601, 701 and the packer ring 602, 702. The two sealing nipples shown differ only in the shape of the O-ring seal carrier 603, 703 which is bonded to the primary sealing element. FIGS. 6B and 7B show the primary sealing element, which fits in the tubing 604, 704 by friction, move against the packer ring when the primary sealing element is energized by the well pressure 605, 705, thus sealing off the well pressure at 606, 706 and 607, 707 from the annulus 608, 708. The packer rings will collapse to their original shape when pressure is removed.
These existing nipples all have their own advantages and disadvantages. One of the main sealing problems is caused by the more frequent use of HYDRILL tubing and its clones in oil and gas wells. An example of HYDRILL tubing is shown in cross section generally at 800 in FIG. 8. It is shown threaded into a dognut 801 which has a back pressure plug thread at 802. This tubing has an entrance diameter 803 which is less than the inside major diameter 804 of the rest of the tubing. In order to get a friction fit seal in the major diameter of the tubing, the sealing material must be able to compress enough to pass through the entrance diameter of the tubing and then expand enough to seal the major diameter of the tubing. The sealing elements proposed in Pitts, Oliver (FIG. 3A), Bullen and McLeod (Nipple Insert), rely on a friction fit in the tubing for sealing and they do not perform well in the HYDRILL type of tubing. There is also the chafing and cutting effects on the seals as they are forced through the back pressure threads in the dognut. The Pitts invention is prone to failure at the elastomer to steel bonded junction due to the treating pressures. The Halliburton design FIG. 2A, is also limited by the strength of the rubber bond between the sealing medium and the nipple body. The McLeod nipple, FIG. 4A, has a measure of safety at this bond. It is also of note that in conventional tubing, if the tubing is out of round or corroded in such a way as to leave a depression, there is the possibility that the sealing medium will not seal, or that it will seal only under the initial well pressure and then rupture under the higher pressures of the well servicing. There is also an economic and operational disadvantage to the bonded sealing elements. They must have the sealing medium stripped off and replaced in an appropriate facility when it is damaged. Designs shown in FIGS. 3A, 5A, 6A and 7A have replaceable sealing elements. The Sutherland-Wenger nipple seal element is initially moved into the sealing position by pressure from the well but in the case of a dead well, this is not available. When in the set position, FIG. 5B, there is a physical limitation on the expansion of the steel ring without it yielding and taking a permanent set. When this happens, it prevents the nipple assembly from being easily withdrawn from the wellhead. There is also a gap in the split ring into which the sealing medium is extruded, and this keeps the split ring from contracting to its original shape, also preventing the nipple assembly from being easily withdrawn from the wellhead.
The Oliver (assigned to Halliburton) nipple of FIG. 3A and the Arrowhead FIG. 6A and Dow-Schlumberger FIG. 7A are prone to failure in the primary seal element bond to the O ring seal carrier as noted in the Dow Schlumberger manual, and there are limitations to the expansion and sealing capabilities of the packer ring, which in case of failure of the primary seal element, must seal both on the nipple body and in the tubing.
All of the mentioned sealing nipples have a sealing capability only while the seal elements are kept under pressure.